IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly. Other protocols are used for media transmission and control, such as Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP).
FIG. 1 illustrates schematically how the IMS fits into the mobile network architecture in the case of a GPRS/PS access network (IMS can of course operate over other access networks). As shown in FIG. 1, the IMS includes a core network and a service network. Call/Session Control Functions (CSCFs) operate as SIP proxies within the IMS core network, and interface with other entities such as Border Gateway Control Functions (BGCFs) and Media Resource Function Controllers (MRFCs) amongst others. A Proxy CSCF (P-CSCF) is the first point of contact within the IMS for a SIP terminal; a Serving CSCF (S-CSCF) provides services to the subscriber; an Interrogating CSCF (I-CSCF) identifies the correct S-CSCF and forwards to that S-CSCF a request received from a SIP terminal via a P-CSCF.
Within the IMS service network, Application Servers (aSs) are provided for implementing IMS service functionality. Application Servers provide services to end users in an IMS system, and may be connected either as end-points over the 3GPP defined Mr interface, or “linked in” by an S-CSCF over the 3GPP defined ISC interface. In the latter case, Initial Filter Criteria (IFC) are used by an S-CSCF to determine which Applications Servers should be “linked in” during a SIP Session establishment (or indeed for the purpose of any SIP method, session or non-session related). The IFCs are received by the S-CSCF from an HSS during the IMS registration procedure as part of a user's Subscriber Profile.
3GPP TS 22.173 (V11.3.0) and 3GPP TS 24.173 (V11.0.0) define the supplementary services that are supported by IMS. For example, the standardized supplementary services supported by IMS include but are not limited to Originating Identification Presentation (OIP), Originating Identification Restriction (OIR), Terminating Identification Presentation (TIP), Terminating Identification Restriction (TIR), Communication Diversion (CDIV), Communication Hold (HOLD), Communication Barring (CB), Message Waiting Indication (MWI), Conference (CONF), Explicit Communication Transfer (ECT), Advice Of Charge (AOC), Communication Waiting (CW), Flexible Alerting (FA), Communication Waiting (CW), Customized Alerting Tones (CAT), and Customized Ringing Signal (CRS). In addition to the standardized supplementary services, the vendor of an IMS Application Server can configure an Application Server so as to implement additional, vendor specific services. An example of such a vendor specific service is the Flexible Communication Distribution service.
The Communications Diversion (CDIV) service, as defined in 3GPP TS 24.604 (V11.0.0), enables a user to divert/re-direct an incoming communication that fulfils certain provisioned or configured conditions to another destination. 3GPP TS 24.604 specifies that, when Communication Diversion occurs, the Application Server (AS) providing the CDIV supplementary service may initiate an announcement towards the calling user in order to inform the calling user about the diversion (see section 4.5.2.6.4). Similarly, 3GPP TS 24.611 (V11.0.0), which defines the Communication Barring (CB) service, also specifies that, when a communication is barred/rejected, the AS providing the CB supplementary service can provide an announcement to the originating user (see sections 4.5.2.4.1, 4.5.2.6.1, and 4.5.2.6.2). In addition, both 3GPP TS 24.604 and 3GPP TS 24.611 state that such an announcement may be played according to procedures as are described in 3GPP TS 24.628.
With regards to 3GPP TS 24.628 (V11.0.0), this document specifies the methods that can be used by a service in order to provide an announcement during the establishment of a communication and when a rejecting a communication request. For example, an AS can use early media, as defined by IETF RFC 3960, for sending the announcement in-band.